let T be _Tree; :: thesis: for a, b, c being Vertex of T holds MiddleVertex a,b,c = MiddleVertex c,b,a
let a, b, c be Vertex of T; :: thesis: MiddleVertex a,b,c = MiddleVertex c,b,a
thus MiddleVertex a,b,c = MiddleVertex c,a,b by Th43
.= MiddleVertex c,b,a by Th40 ; :: thesis: verum